Remote trace gas detection and analysis

ABSTRACT

A system for the remote detection and analysis of trace chemical agents in the air. A beam of electromagnetic radiation is used to radiate a cloud. The radiation energy that is absorbed by the cloud is thermalized by collisional energy transfer between the molecules that absorb the radiation. Increases in the cloud temperature increase the emission intensity of the molecules against the background, resulting in improved detection of the target molecules. A tracking telescope is used to collect the thermal emissions generated by the radiation beam. A spectrometer is used to resolve the emissions from the cloud and generate an emissions spectrum. The wavelength of the electromagnetic radiation can be selected to be in resonance with the absorption lines of water or oxygen molecules in the cloud, or to be in resonance with absorption lines of known target molecules in the cloud to generate the heat.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method for detecting trace gasesin the air and, more particularly, to a method of radiating a chemicalcloud to heat the cloud and increase its temperature relative to thebackground, and then detecting chemicals in the cloud by spectroscopy.

2. Discussion of the Related Art

It is known in the art to detect certain constituents in a chemicalcloud in the air by spectral analysis of the molecules making up thecloud. This type of chemical detection has many applications, includingdetecting natural gas leaks from underground pipes, chemical clouds fromchemical spills, volatile organic vapor (VOC) from chemical processes,pollution from smoke stacks and the like, military chemical warfareagents, and other toxic gases present in the air. Typically, this typeof spectral analysis of a chemical cloud is performed remotely,sometimes up to 10-20 km away, because the constituents in the cloud maybe toxic, and thus a threat to health, or it may not be possible todirectly detect the chemical cloud. The distance the detectinginstrument has to be from the cloud for this remote type of passivesensing depends on the particular application, and different systemsexist for different applications.

To perform this type of detection and analysis, a spectrometer, such asa Fourier transform infrared (FTIR) spectrometer, is directed towardsthe chemical agent cloud from a remote location, so that it passivelyreceives emissions therefrom. If the chemical cloud is warmer than thebackground, such as sky, mountains, or other terrain, along thefield-of-view of the spectrometer, target molecules in the cloud willexhibit emissions having an energy greater than the background emissionsfrom the sky. If the chemical cloud is the same temperature as the sky,the target molecules within the cloud are absorbing photons at the samerate that they are emitting photons, so that there is no net energyexchange between the cloud and the background, and no differencerelative to the background. As the temperature of the cloud increases,more photons are released from the chemicals in the cloud, which areavailable to be received by the spectrometer.

The spectral display generated by the spectrometer from the emissionsprovides emission lines and bands at certain wavelengths that isindicative of the atoms and molecules in the cloud. Because eachmaterial has its own spectral “fingerprint” representative of itsmolecules, the detected spectral display can be compared to a known“fingerprint” of a particular chemical to determine if that chemicalexists in the cloud.

A problem exists with the passive remote sensing techniques that arecurrently used in the art because the temperature difference between thechemical cloud and the sky is often very small. In many cases, thetemperature of the cloud is only about 2-3° C. warmer than thetemperature of the background. Because of such a small temperaturedifference, the detectable emissions from the cloud is typically veryweak. This results in a poor signal-to-noise ratio, and thus poordetection sensitivity and possibly a high false alarm rate.

What is needed is a remote chemical detection system that causes thechemical cloud to be heated so that the temperature of the cloud issignificantly different than the background. It is therefore an objectof the present invention to provide a remote chemical detection systemof this type.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a techniquefor the remote detection and analysis of trace chemicals in the air isdisclosed. A beam of electromagnetic radiation from an electromagneticradiation source is used to radiate a suspected chemical cloud. Theradiation energy that is absorbed by the cloud is quickly thermalizeddue to a rapid collision energy transfer between the molecules thatabsorb the radiation and the surrounding air molecules. This collisionalenergy redistribution will result in heating the chemicals in the cloud.An increase in the temperature of the cloud will increase the emissionintensity of the molecules against the background, resulting in animprovement in the detection of the chemicals.

A tracking telescope is then used to collect the thermal emissions ofthe target molecules generated by the radiation. The tracking telescopecan be located in the vicinity of the electromagnetic radiation sourcesuch that the viewing axis of the telescope is preferably coaxial withthe propagation direction of the electromagnetic radiation. Aspectrometer, such as an FTIR spectrometer, is used to resolve theemissions from the cloud that are enhanced by the radiation and togenerate an emissions spectrum. The emissions spectrum is used toidentify suspect molecules in the cloud by comparing the detectedemissions to the known “fingerprint” vibrational spectrum of the suspectmolecules.

The electromagnetic radiation source, telescope and FTIR spectrometercan be housed on a platform to scan over a wide area for surveillance ofchemical clouds. Alternatively, the tracking telescope and the FTIRspectrometer can be located at a separate location from that of theelectromagnetic radiation source. The viewing axis of the telescope willintersect with the beam of the electromagnetic beam. In this arrangementthe location of the chemical cloud can be determined based on theintersection of the electromagnetic radiation beam and the view axis ofthe telescope.

The electromagnetic radiation can be microwave, millimeter wave,infrared, visible, or ultraviolet radiation. The wavelength of theelectromagnetic radiation can be selected to be in resonance with theabsorption lines of the chemicals, or of water vapor or oxygen moleculesthat are commonly present in the cloud. If the wavelength of theelectromagnetic radiation is chosen to be in resonance with theabsorption lines of the target chemical molecules, the returned emissionintensity, as a function of the excitation wavelength, can be used toprovide an increased discrimination of the chemicals against possibleinterference background chemicals.

Additional objects, features and advantages of the present inventionwill become apparent from the following description and appended claimstaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a remote spectral analysis sensing system,according to a preferred embodiment of the present invention;

FIGS. 2(a)-2(d) show the spectral footprint of four different chemicalagents; and

FIG. 3 is a graph with wavelength on the horizontal axis and theabsorption of the agent VX and output intensity of a CO₂ laser on thevertical axis showing the overlapping of a CO₂ laser with absorptionspectrum of the agent VX;

FIG. 4 is a graph with wavelength on the horizontal axis and absorptionon the vertical axis showing absorption spectrum of sarin relative to aCO₂ laser tuning range; and

FIG. 5 is a graph with frequency on the horizontal axis and absorptioncoefficient on the vertical axis showing microwave absorption fromatmospheric gases.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following discussion of the preferred embodiments directed to atechnique for the remote sensing of a chemical cloud is merely exemplaryin nature, and is in no way intended to limit the invention or itsapplications or uses.

FIG. 1 is a plan view of a spectral sensing system 10, according to anembodiment of the present invention. The system 10 includes a radiationsource 12, a telescope 14 and a spectrometer 16. The radiation source 12can be any suitable laser or microwave source consistent with thediscussion herein. The telescope 14 is a Newtonian type telescope,including a collecting mirror 18 for receiving radiation from a sceneand a turning mirror 20. However, the telescope 14 can be any suitabletelescope for receiving and focusing radiation from a scene consistentwith the discussion herein. The spectrometer 16 can be any type ofspectral detecting device that provides a spectral display over apredetermined spectrum, such as an FTIR spectrometer, an acousto-opticspectrometer, or a grating dispersed spectrometer. The source 12,telescope 14 and spectrometer 16 can be mounted within a suitablehousing, and can be included on a platform capable for scanning over awide area for increased surveillance. Additionally, the system 10 can bemade compact and portable to be readily moved from place to place.

The radiation source 12 emits a radiation beam 22 towards a pair ofturning mirrors 24 and 26 to align the beam 22 to be near co-linear withradiation received by the telescope 14 from the scene. The radiationbeam 22 can be microwave, millimeter wave, infrared, visible orultraviolet radiation, depending on the particular application andsystem being used, as will be discussed below. The radiation beam 22 isdirected towards a suspected chemical cloud 28 to excite moleculeswithin the cloud 28 to cause heat to be generated to increase thecloud's temperature relative to the background, usually sky, mountainsor other terrain. In alternate embodiments, the radiation beam 22 can bedirected towards the cloud 28 in a manner that is not co-linear with theemissions from the cloud 28. In this type of design, the location of thecloud 28 can also be determined as the intersection point between theradiation beam 22 and the field of view of the telescope 14.

The source 12 can be selected so that the wavelength of the radiationbeam 22 is in resonance with a particular target molecule or moleculesexisting in the cloud being detected. The wavelength of the radiationbeam 22 can also be selected to be in resonance with the absorptionlines of water vapor or oxygen molecules commonly present in air. Theresonance causes the target molecules, water vapor or oxygen moleculesto rotate or vibrate which causes their energy to increase. Also,electrical transistions may occur in the molecules. The radiation energyabsorbed by the water vapor, the oxygen molecules or the targetmolecules in the cloud 28 is thermalized due to collision energytransfer causing inter-molecular relaxation. At atmospheric pressure,this thermalization is very rapid. This collisional energyredistribution results in heating the molecules in the cloud 28. Anincrease in the temperature of the cloud 28 will increase the emissionintensity of the molecules in the cloud against the background,resulting in an improved detection of the molecules. If the wavelengthof the electromagnetic radiation is chosen to be in resonance with theabsorption lines of the target molecules, the returned emissionintensity, as a function of the excitation wavelength, can be used toprovide an additional way for discrimination against possibleinterference background chemicals. This is because the returned emissionintensity from the target molecules should increase substantially as theexcitation is tuned to the resonance absorption lines of the targetmolecules. In the contrast, the emission intensity from backgroundchemicals should not increase appreciably at these excitationwavelengths.

The telescope 14 collects thermal emission returned from the cloud 28.The spectral content of the emission is then analyzed by thespectrometer 16. The emission spectrum, typical in the 8-14 micronregion, is used to identify the molecules in the cloud 28 by comparingthe detected emissions to the known “fingerprint” vibrational spectrumof predetermined molecules. Alternatively, an imaging spectrometer, suchas a hyperspectrometer imager, can be used to obtain spatially resolvedspectrum. The contrast from the spatially resolved spectrum can furtherbe used to discriminate against any other interference backgroundchemicals that may be present.

Several electromagnetic radiation sources, including infrared lasers,such as a CO₂ laser and a DF laser, and microwaves may be used as theradiation source. The CO₂ laser is a preferred excitation source fordetection of chemical agent cloud. This is because severalphosphonate-type chemical agents, including sarin (GB), soman (GD),tabun (GA) and VX, as shown in FIG. 2, have absorption bands in the 9-10μ region that can be excited by a CO₂ laser.

FIG. 3 illustrates some overlapping of CO₂ laser lines with theabsorption spectrum of agent VX. The upper curve shows the absorptionspectrum of agent VX and the dots on the curve show the overlapping ofthe absorption curve with relatively strong ¹²CO₂ laser lines. The lowercurve shows the intensity distribution of a typical tuned ¹²CO₂ laser.The strong peaks of the VX vapor can be excited by the p-branch of the9.6-μm band or 10.6-μm band of a ¹²CO₂ laser. A single strong CO₂ laserline that overlaps with the strong absorption band may be employed forinitial searching of the target chemicals. Further identification oftarget molecules can be achieved by comparing the returned emission atdifferent CO₂ laser lines against the “fingerprint” absorption spectrumof the target chemicals.

FIG. 4 shows another example of the overlapping of sarin (agent GB) withCO₂ laser, including both isotopes of ¹²CO₂ and ¹³CO₂. This illustratesthat strong absorption bands of the chemicals can be excited by adifferent isotope of a CO₂ laser.

Table 1 shows an estimate of the power required for heating a GB vaporcloud by 10° C. This assumes that a CO₂ laser is tuned near the peakabsorption of the vapor, the concentration of the GB is assumed to beabout 100 ppm in the air, the laser beam on the target cloud is about5-cm diameter, and the duration of irradiation is about 1 second. Table1 shows the required CO₂ laser power is relatively low, about 28 W,mainly because of relatively strong absorption cross-sections of thephosphonate bands in the 9-10μ region. In addition, the range from thesource to the target cloud can be quite long, exceeding more than 5 km,mainly because the air is practically transparent to the CO₂ laser inthe range of concern.

The DF laser, as well some HF lasers, may also be used as an excitationsource. As shown in FIG. 2, many of the chemical agents have absorptionbands near the region that overlap with either the DF or HF laser lines.Table 1 shows that the required DF laser for heating a GB vapor cloud of100 ppm in the air by 10° C. is relatively mild, about 137 W for aduration of 1 second. The estimate again assumes the laser beam on thetarget cloud is about 5-cm diameter.

When CO₂ lasers are used as an excitation source, strong scatteredintensity from the CO₂ laser may interfere with the FTIR spectralmeasurements. It is desirable to turn on the laser for a certainduration and then turn off the laser momentary during the FTIR spectralmeasurements. Although the CO₂ laser is turned off momentary, thechemical cloud won't cool off immediately, as it takes some time to loseits heat to the surrounding air.

Microwave radiation may also be used to heat a chemical cloud. Oneconvenient way is to heat the cloud via the excitation of the O₂ or H₂Ovapor that is present in the cloud. The energy absorbed by O₂ or H₂O canbe thermalized rapidly and heat the cloud. Hence, in this approach, achemical cloud can be heated without prior knowledge of the constituentsand their absorption spectra. FIG. 5 shows the absorption bands of O₂ orH₂O in air from about 15 GHz to 350 GHz. A FTIR spectrometer can be usedto identify the chemical constituents by spectral analyzing the returnedradiation in the infrared region from about 8 to 14 μm. A microwavesource in conjunction with a spectrometer is like providing an“universal detector” capable of detecting various kinds of chemicalvapor. However, a relatively high power microwave may be needed since asubstantial fraction of the power is absorbed by the O₂ or H₂O vapor intransmission through the air before reaching the target.

An alternative way is to excite the chemicals directly by tuning themicrowave frequency in resonance with the chemical absorption lines. Theabsorption coefficients for chemical warfare agents are generally notknown too well. However, several of other molecules, such as H₂S, SO₂,and CH₃I have been reported. See, for example, N. Gopalsami, S.Bakhtiari, A. C. Raptis, S. L. Dieckman, and DeLucia, “Millimeter-waveMeasurements of Molecular Spectra with Application to EnvironmentalMonitoring,” IEEE Transactions on Instrumentation and Measurement, Vol.45, p. 225-230, 1996.

Microwave or millimeter wave radiation has a very different spectralregion than infrared radiation, which is the emissions range usuallyanalyzed by the spectrometer 16. Therefore, minimal interference in themeasurements from using microwave or millimeter wave radiation isexpected. The spectral range of interest for analyzing the returnedemissions is typically between 8-14 microns, where many chemical vapors,such as certain chemical agents, emit radiation.

Although the path between the cloud and the sensor unit is also heated,if the excitation via water vapor or oxygen absorption lines is chosen,the return emission is expected to have minimal absorption by the columnof the heated air. This is because the absorption by air is quite weakin this spectral region of interest. Although there are weak CO₂absorption bands in the 8-14 micron region due to the hot-bandabsorption from the μ₂ and 2μ₂ levels of CO₂, these bands are normallyfairly weak near ambient temperature. It is anticipated that theseabsorption bands will not cause any serious interference of the returnedsignal in the spectral range of 8 to 14 μm of interest, if thetemperature rise is kept below 10 to 20° C.

The required microwave power for the source 12 depends upon thenecessary temperature difference between the cloud 28 and thebackground, the range of the cloud 28, and the absorption coefficientand concentration of the absorber. Table 1 shows an estimate of therequired microwave power and range for heating a 100-ppm GB vapor cloudby 10° C., via absorption of O₂ at about 60 GHz, H₂O at 22.3 GHz, and GBvapor at 90 GHz, for a duration of 1 second. The absorptioncross-section of O₂ and H₂O are from Ulaby, et al. The absorption crosssection for GB is taking to be an average value of that of H₂S, SO₂, andCH₃I cited in the above reference by Gopalsami, et al. It is assumedthat the beam 22 can be focused to a spot size of 5-cm diameter at a 200meter range and 10-cm diameter at a 5-km range, and that the source 12is on for 1 second.

There are trade-offs among choice of different absorbers. It appearsthat O₂ yields a relative low required power of about 16 kW mainlycaused by relatively high absorption of O₂ in the air. However, therange is likely limited to 200 m. On the other hands, H₂O yields asubstantially high required energy of about 270 kW, although the rangecan exceed 5 km. GB vapor may not be a good choice of absorber becauseof exceedingly high required microwave power. Of course, the requiredmicrowave power can be substantially reduced if the irradiation can beextended over a longer duration.

TABLE 1 Required Laser of Microwave Power for Heating 100-ppm GBchemical Cloud by 10° for 1 second duration Excitation source CO₂ laserDF laser Microwave Microwave Microwave Wavelength ˜9.6 micron ˜3.58micron ˜60 GHz ˜22.3 GHz ˜90 GHz Absorbing medium GB vapor cloud GBvapor cloud Oxygen Watervapor(30% H₂O) GB vapor cloud Cross section, cm²3.4 × 10⁻¹⁸ 0.7 × 10⁻¹⁸ 4.9 × 10⁻²⁴ 1.9 × 10⁻²⁴ 1.0 × 10⁻²¹ Beamdiameter 5 cm 5 cm 5 cm 10 cm 10 cm Range 0-5 Km 0-5 Km 0-200 m 0-5 Km0-5 Km Required Power 28 W 137 W 16 KW 270 KW 380 KW

One article has investigated the relationship between improvements indetection sensitivity and the temperature difference between the targetvapor cloud and the background temperature. See T. C. Gruber and J.Ditillo, “Quality assurance measurements for passive FTIR datacollection,” AAPCA Specialty Conference Proceedings SP Optical RemoteSensing for Environmental and Process Monitoring Proceedings of the 1995Specialty Conference, Sep. 25-27, 1995. The quality in the returnedemission spectrum was expressed as a “score”. For temperatures closer toor below 2° C., the score is practically zero. Once the temperaturepasses a threshold value of ˜2° C., the score increases markedly withincreasing temperature difference. Hence, an increase in the temperaturedifference by 10° C. or higher, by means of the laser or microwaveheating, provides an improvement in the sensitivity of detecting thesemolecules substantially.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. A system for the detection and analysis of tracechemicals in an aggregate cloud, said system comprising: a radiationsource, said radiation source directing a radiation beam towards thecloud, said radiation beam thermalizing the cloud to raise a bulk gastemperature of the cloud against its surrounding background; receivingoptics for receiving emissions from the cloud as a result of the cloudhaving a higher temperature than the background; and a spectrometerresponsive to the emissions from the receiving optics, said spectrometergenerating a spectral display of certain constituents in the cloud. 2.The system according to claim 1 wherein the spectrometer is selectedfrom the group consisting of FTIR spectrometers, acousto-opticspectrometers and grating dispersion spectrometers.
 3. The systemaccording to claim 1 wherein the receiving optics are part of atelescope.
 4. A system for the detection and analysis of trace chemicalsin an aggregate cloud, said system comprising: a radiation source, saidradiation source directing a radiation beam towards the cloud, saidradiation beam thermalizing the cloud to raise a bulk gas temperature ofthe cloud against its surrounding background, said radiation sourcegenerating a radiation beam selected from the group consisting ofmicrowave, millimeter wave, infrared, visible, and ultraviolet radiationbeams; receiving optics for receiving emissions from the cloud as aresult of the cloud having a higher temperature than the background; anda spectrometer responsive to the emissions from the receiving optics,said spectrometer generating a spectral display of certain constituentsin the cloud.
 5. A system for the detection and analysis of tracechemicals in an aggregate cloud, said system comprising: a radiationsource, said radiation source directing a radiation beam towards thecloud, said radiation beam thermalizing the cloud to raise a bulk gastemperature of the cloud against its surrounding background; receivingoptics for receiving emissions from the cloud as a result of the cloudhaving a higher temperature than the background; directional opticspositioned to receive the radiation beam from the radiation source anddirect the radiation along a direction substantially co-linear with thedirection of the emissions received from the cloud; and a spectrometerresponsive to the emissions from the receiving optics, said spectrometergenerating a spectral display of certain constituents in the cloud.
 6. Asystem for the detection and analysis of trace chemicals in an aggregatecloud, said system comprising: a radiation source, said radiation sourcedirecting a radiation beam towards the cloud and generating a radiationbeam having a wavelength that is in resonance with a particular targetmolecule existing in the cloud, said radiation beam thermalizing thecloud to raise a bulk gas temperature of the cloud against itssurrounding background; receiving optics for receiving emissions fromthe cloud as a result of the cloud having a higher temperature than thebackground; and a spectrometer responsive to the emissions from thereceiving optics, said spectrometer generating a spectral display ofcertain constituents in the cloud.
 7. A system for the detection andanalysis of trace chemicals in an aggregate cloud, said systemcomprising: a radiation source, said radiation source directing aradiation beam towards the cloud and generating a radiation beam havinga wavelength that is in resonance with absorption lines of at least oneof water vapor and oxygen molecules existing in the cloud, saidradiation basin thermalizing the cloud to raise a bulk gas temperatureof the cloud against its surrounding background; receiving optics forreceiving emissions from the cloud as a result of the cloud having ahigher temperature than the background; and a spectrometer responsive tothe emissions from the receiving optics, said spectrometer generating aspectral display of certain constituents in the cloud.
 8. A system fordetecting a chemical agent in a chemical cloud against a sky background,said system comprising: a radiation source, said radiation sourcedirecting a radiation beam towards the cloud to thermalize and heatconstituents in the cloud and raise its temperature relative to thetemperature of the background, said increase in the temperature of thecloud enhancing passive emissions from the cloud: a telescope responsiveto the emissions from the cloud as a result of the cloud having a highertemperature than the background, said telescope focusing and directingthe emissions; and a spectrometer responsive to the emissions from thetelescope, said spectrometer generating a spectral display of theconstituents in the cloud.
 9. The system according to claim 8 whereinthe radiation source generates a radiation beam having a wavelength thatis in resonance with absorption lines of at least one of water vapor andoxygen molecules existing in the cloud.
 10. The system according toclaim 8 wherein the radiation source generates a radiation beam selectedfrom the group consisting of microwave, millimeter wave, infrared,visible, and ultraviolet radiation beams.
 11. The system according toclaim 8 wherein the spectrometer is selected from the group consistingof FTIR spectrometers, acousto-optic spectrometers and dispersionspectrometers.
 12. A system for detecting a chemical agent in a chemicalcloud against a sky background, said system comprising: a radiationsource generating a radiation beam having a wavelength that is inresonance with a particular target molecule existing in the cloud, saidradiation source directing a radiation beam towards the cloud tothermalize and heat constituents in the cloud and raise its temperaturerelative to the temperature of the background, said increase in thetemperature of the cloud enhancing passive emissions from the cloud; atelescope responsive to the emissions from the cloud as a result of thecloud having a higher temperature than the background, said telescopefocusing and directing the emissions; and a spectrometer responsive tothe emissions from the telescope, said spectrometer generating aspectral display of the constituents in the cloud.
 13. A system fordetecting a chemical agent in a chemical cloud against a sky background,said system comprising: a radiation source, said radiation sourcedirecting a radiation beam towards the cloud to thermalize the cloud andraise its temperature relative to the temperature of the background,said increase in the temperature of the cloud enhancing passiveemissions from the cloud; a telescope responsive to the emissions fromthe cloud as a result of the cloud having a higher temperature than thebackground, said telescope focusing and directing the emissions:directional optics positioned to receive the radiation beam from theradiation source and direct the radiation along a directionsubstantially co-linear with the direction of the emissions receivedfrom the cloud; and a spectrometer responsive to the emissions from thetelescope, said spectrometer generating a spectral display of theconstituents in the cloud.
 14. A method of detecting a chemical agent ina gas cloud against a background, said method comprising the steps of:heating the gas cloud relative to the background by directing a beam ofradiation towards the cloud to thermalize the cloud to raise a bulk gastemperature of the cloud against its surrounding background and increaseemissions from the cloud; receiving the emissions from the cloud as aresult of the cloud having a higher temperature than the background: andgenerating a spectral display indicative of the constituents in thecloud.
 15. The method according to claim 14 wherein the step of heatingthe cloud includes using a radiation beam having a wavelength that is inresonance with a particular target molecule existing in the cloud. 16.The method according to claim 14 wherein the step of heating the cloudincludes using a radiation beam having a wavelength that is in resonancewith absorption lines of at least one of water vapor and oxygenmolecules existing in the cloud.
 17. The method according to claim 14wherein the step of directing the radiation beam includes directing theradiation beam to be co-linear with the direction of the emissionsreceived from the cloud.
 18. A method of detecting a chemical agent in agas cloud against a background, said method comprising the steps of:heating the gas cloud relative to the background by directing a beam ofradiation having a wavelength predominantly from the group consisting ofmicrowave, millimeterwave, infrared, visible, and ultravioletwavelengths towards the cloud to excite constituents in the cloud toraise a bulk gas temperature of the cloud against its surroundingbackground and increase emissions from the cloud; receiving theemissions from the cloud as a result of the cloud having a highertemperature than the background; and generating a spectral displayindicative of the constituents in the cloud.